Move from 6x2TB to 6x3TB

The Pro with 10TB won't be able to completely back up the Pro 6 with 15TB, but at least the most critical 2/3 would be covered. Then as time an finances permit, you can slowly replace the 2TB drives in the Pro with 3TB. Since you will always have a current and complete backup with the second NAS, you can factory default either if needed without losing any data or having to make the extra effort to backup a massive array. The time taken to backup up 10TB to external drives would be days not hours. Even using NFS and a backup job on the Pro to back up all data to the Pro 6, you will be looking at more that one day, maybe close to two. However, once that initial backup is done, rsyncing the files between the two units the time will be measured in minutes.

Rsync is not like the traditional backup where you create an initial backup and incrementals for a period, and then create a new initial and then do incrementals again. Rsync actually synchronizes the files between the two units so that once the rsync is done, the file on the target is identical to the one on the source. By default the check box to erase files on the target if the file has been deleted on the source is unchecked. This makes sure that if a file is erased accidentally then it stays on the target. On member leaves it unchecked and then once a week, checks the box and runs the job to delete obsolete files. I leave mine checked. There have been several situations where I deleted a file or messed up a file, and then deleted it and simply copied the file back over from the backup. (It is there until the back up jobs run at midnight).

There is just no better, faster way to back up a large array on an NAS than to another NAS, even if that array is slightly smaller. I used to back up my NAS to external drives and it was a PITA. Now, I no longer worry about my backups and have they been updated recently.